This report describes the electrochemical behavior of polyaniline prepared in aqueous solution with various acid electrolytes. Electroactive polyaniline with two different cyclic voltammograms, i.e., with and without the additional peak at 500 mV, can be produced by controlling the current densities during the preparation. The oxidation reaction is coulombically reversible, and the rate depends on the acidity of the solution but is independent of the nature of the counter anion. In solutions more acidic than 0.1M, the charge is linear with time in the initial period. In 10-3M acid solutions, the charge has a t1/2 dependence. The films are also electroactive in nonaqueous solvents, and voltammograms with well-defined waves result when the solution contains both organic salt and protic acid electrolyte. Polyaniline is stable in the dry state but not in aqueous electrolyte. TGA shows that the material dehydrates at 80°C and completely decomposes at 185°C. SEM analyses show that the initially deposited film is a continuous film and that the subsequent deposit has a fibril structure. © 1988, The Electrochemical Society, Inc. All rights reserved.